Encapsulant with fluxing properties and method of use in flip-chip surface mount reflow soldering

ABSTRACT

Encapsulated electrical component assemblies and methods of electrically connecting an electrical component having a plurality of component electrical terminations to a component carrying substrate having a plurality of substrate electrical terminations at surface mount reflow soldering conditions is described. The electrical and substrate components have an encapsulant-forming composition sandwiched therebetween and encasing said pluralities of component and substrate electrical connections. The described invention relates to using an encapsulant-forming composition comprising a thermosetting resin (preferably an epoxy resin) and a cross-linking agent (preferably an anhdride) for said resin that cross-links said resin and that also acts as a fluxing agent and optionally includes a catalyst for initiating cross-linking at required conditions. The gel point of the encapsulant-forming composition is reached after solder melt.

This application is a continuation-in-part of Application Ser. No.08/514,049, filed Aug. 11, 1995, now abandoned, the disclosure of whichis incorporated herein by reference thereto.

DESCRIPTION Technical Field

The invention relates generally to electrical interconnection ofelectrical components to substrates, in particular flip-chip reflowsoldering and specifically the development of encapsulant-formingcompositions.

BACKGROUND OF THE INVENTION

Epoxy resin compositions have been used as semiconductor deviceencapsulants for over 25 years as noted by reference to U.S. Pat. No.3,449,641, granted Jun. 10, 1969.

Anhydride-cured epoxy resin encapsulants used in flip-chip manufacturingmethods are described in U.S. Pat. No. 4,999,699, granted Mar. 12, 1991,and U.S. Pat. No. 5,250,848, granted Oct. 5, 1993. Encapsulant-formingcompositions are applied after electrical interconnection.

The application of an encapsulant-forming (encapsulating) compositionprior to interconnection by reflow soldering, wherein electricalinterconnection occurs in the presence of an encapsulating compositionis described in U.S. Pat. No. 5,128,746, granted Jul. 7, 1992. In U.S.Pat. No. 5,128,746, flip-chip production methods are described whereelectrical interconnection is achieved by adding a fluxing agent to amixture of epoxy resin and curing agent prior to cure. During reflowsoldering, the fluxing agent is activated and the resin is cured.

The use in the prior art of cross-linking agents having flux propertiesis found in PCT International Publication Number WO 93/06943, publishedApr. 15, 1993 and its U.S. counterpart, U.S. Pat. No. 5,376,403, grantedDec. 27, 1994. In the publication, enhanced sintering is described usinga protected cross-linking agent with fluxing properties in a metalpowder and epoxy resin system where the solder is used to form theconductive film. Solder powder addition is used to sinter the metalpowder, typically copper or silver, before setting the resin in order tocreate solid electrically conductive bridges between the powdered metalparticles.

The use of synthetic thermosetting polymer resins together withsoldering flux agents is described in U.S. Pat. No. 3,791,027, grantedFeb. 12, 1974. Therein epoxide resin compositions are described whereinfluxing agents react with the epoxide resin to strengthen solder joints.

Electrically conductive adhesive compositions in which solder powder, achemically protected cross-linking agent with fluxing properties and areactive monomer or polymer (inclusive of epoxy resins) are described inU.S. Pat. No. 5,376,403, granted Dec. 27, 1994.

In the present invention, the fluxing additive of U.S. Pat. No.5,128,746 is eliminated, while the function of fluxing is retained byselecting a cross-linking agent that has the property of also operatingas a fluxing agent. In the flip-chip production method of the presentinvention, where electrical interconnection occurs within theencapsulating composition, the preferred embodiment of the inventionresides in selecting a combination of dual functioning cross-linkingagent and thermosetting resin or combination of such agent andthermosetting resin with a selected catalyst and controlling thesequence of flow soldering and gel formation to avoid inhibition ofsoldering. This is accomplished by providing an encapsulant composition,which, at surface mount reflow profile conditions, in which the gelformation (gel point is reached) after solder melt; that is, the gel isformed/gelation occurs after reflow soldering, whereby soldering is notinhibited.

As noted by reference to “MANUFACTURING TECHNIQUES FOR SURFACE MOUNTEDASSEMBLIES,” Wassink, R. K. and Verguld, M. F., 1995 ELECTROCHEMICALPUBLICATIONS, LTD., soldering methods (and equipment) have convergedfrom various IR soldering concepts to one main method, namely,hot-convection soldering. Besides this method other methods are used,but only in specific cases, such as resistance soldering for outer leadbonding of TAB and for soldering on foils.

In Wassink et al., at pages 275, 276, a typical profile for reflowsoldering is described. A hot-air convection soldering oven having anumber of zones whose temperature can be controlled separately is usedin order to attain the desired temperature profile along the length ofthe entire oven. Such profile enables all joint areas to reach thesoldering temperatures with limited temperature differences between thejoint areas of components with different thermal mass.

Wassink et al. describes the typical three step heating approach of theprior art frequently used in reflow soldering using multiple hot-airconvection ovens.

As also described in Wassink et al., the three steps are:

-   -   (i) starting with rapid heating to bring heat into the product        (this reduces the length of the oven);    -   (ii) second step concerns temperature equalizing, i.e., to        reduce the temperature differentials; usually a kind of        temperature plateau for the hottest parts is pursued while the        temperature rise of the coldest parts is chosen to be relatively        slow; the effectiveness of this step can easily be assessed by        the temperature differentials that exist on the assembly just        before it enters the next step;    -   (iii) final rapid heating and subsequent cooling.

As further described in Wassink et al., each limit of the profile isdetermined by the maximum allowable thermal load of one of the parts ofthe assembly to be soldered.

-   -   The maximum (peak) temperature is determined by the base        material of the printed board. Higher temperatures than 280° C.        will cause delamination. (Note: In most cases the printed board        is the hottest component.)    -   The minimum soldering (peak) temperature is determined by the        wetting of component metallizations.    -   The maximum time and temperature of the equalize region is        determined by the solder paste. In the case of too heavy a        thermal treatment, the activator (flux) in the solder paste will        be consumed already at this stage of the process.    -   The time for which the solder is in the molten stage (in        combination with a maximum temperature) is restricted by the        formation of intermetallic layers inside the soldering joint.        These layers make the soldered joint more brittle.

The specific values of the mentioned boundaries are determined based onthe components and board material used.

A surface mount reflow profile for a 63 Sn/37 Pb solder illustrating thetypical ranges is shown in FIG. 4.

In U.S. Pat. No. 3,791,027 (“Angelo”), the disclosure of which isincorporated herein by reference thereto, polymers and other materialswhich contain chemical functionalities; such as, amide, amino, carboxyl,imino, and mercaptan; which serve as flux agents are described. Whensoldering metals, these materials can be combined with materials whichcontain other functionalities; such as, epoxide and isocyanate toproduce thermosetting polymers. Angelo describes three polymercategories in his invention which are set forth below.

-   1. Chemical functionalities; such as, carboxy terminated    polybutadiene and carboxy terminated polyisobutylene, which, when    used alone, do not harden and are easily removed with solvents.    These are in essence fluxes and contain the same chemical    functionalities found in traditional soldering fluxes.-   2. Formulations that are non-crosslinking and can be softened or    melted with the addition of heat. Examples cited in Angelo include    Versarid 712 and Acryloidat 70. Since cross-linking does not take    place, these formulations are similar to standard rosin or resin    based fluxes frequently used in reflow soldering which contain    chemical functionalities such as amino, carboxyl, amide, etc. Thus    the same chemical functionalities are present both traditional rosin    and resin fluxes and in the examples cited in Angelo which do not    chemically cross-link to form thermoset polymer and hence may be    removed by using a solvent or may be reheated and remelted to enable    resoldering of solder joints.-   3. Combinations of materials, which contain the chemical    functionalities necessary to promote solder wetting; such as,    carboxy, amino, etc., and materials that react chemically to form    thermosetting polymers that cannot be easily removed through use of    a solvent or reheated and remelted. Specifically Angelo shows    examples of combinations of materials which contain such    functionalities with epoxy resin materials which, when heated, form    cross-linked networks which are not easily removable or cannot be    remelted. Angelo cites the usefulness of such combinations to reside    in their ability to reinforce the strength of the solder joint in    situations when there is a low probability that a solder joint will    need to be resoldered.

Pennisi, et al., U. S. Pat. No. 5,128,746, also describes the use ofcombinations of materials which contain chemical functionalities knownto serve as fluxes and materials; such as, epoxy resins, which whenreacted with the addition of heat, form chemically cross-linked polymerswhich add strength to solder joints and are not easily removable.Although Pennisi describes the function of the epoxy thermoset polymersas providing environmental protection to the flip-chip, the epoxyencapsulant described by Pennisi is also known to strengthen the fragilesolder joints. Pennisi lists flux agents; such as, malic acid and otherdicarboxylic acids that remove metal oxides and promote solder wetting.In essence, a material, malic acid, containing the carboxyl functionalgroup, which is known to promote solder wetting, is combined withmaterials, epoxy resins, that form cross-linking, thermoset polymers.

In a third example, described in Capote, U.S. Pat. No. 5,376,403, amaterial containing a chemical functionality, such as carboxyl, known toassist in solder wetting, is combined with materials that formcross-linking thermoset polymers that are used in ink systems thatassist in the fusing of low melting alloy powders with high meltingmetals and assist in the adhesion of the resultant metal network to asubstrate.

In each case (Angelo, Pennisi, Capote) in which a material containing achemical functionality, known to promote solder wetting, is combinedwith materials such as epoxy resins that form thermoset polymers, amethod of heating is described in which the assembly is heated rapidlyabove the solder melt point. The application of temperatures above thesolder melt point 183° C. is critical as the solder must liquify inorder to wet the surface metal.

As thermosetting polymers are initiated by the application of heat inorder to stimulate cross-linking reactions, it becomes necessary tounderstand the cure kinetics involved in the curing of the materialcombinations selected. By chemically protecting the cross-linkingmaterial of the combination, Capote ensures that the cross-linkingreactions are delayed and appropriate during the rapid heating processdescribed in his invention.

Similar heating methods are described in Angelo and Pennisi, who bothdescribe the application of heat during the soldering process as quickand rapid. As described previously in Wassink et al., a three stepheating profile is typically used to solder electronic components tosubstrate boards. Rapid heating, as called for in Angelo, Pennisi andCapote, would adversely affect the parts and assemblies duringsoldering. This includes damage to components at high thermal excursionrates.

Thus, one frequently finds the heating step to be done using multizoneovens which allow materials in assemblies to achieve thermal equilibriumat temperatures above room temperature but lower than the solder meltpoint (183° C.) in order to reduce thermal shock and subsequent damage.In SMT, this heating process in known as a surface mount reflow profile.

Therefore, in using combinations, as set forth by Angelo, Pennisi andCapote, in which materials containing chemical functionalities; such ascarboxyl and amino, known to promote solder wetting, are combined withmaterials that form cross-linked thermoset polymers through the additionof the heat, heating processes are used that do not involve a rapidheating rate to the solder temperature but instead allow materials to beused in the final assembly to reach a thermal equilibrium. Above roomtemperature but lower than soldering temperature, it becomes critical tounderstand the cure kinetics of the combination of thermosettingmaterials, in view of the desired non-rapid heating profile in order toprevent significant crosslinking of the combination prior to solder meltpoint.

The Present Invention

The present invention relates to thermosetting resin encapsulatingformulations in which the cross-linking agent functions as flux forreflow soldering. Included in such formulation are systems in which thecross-linking of resin with such dual purpose cross-linking agent iseffected by a catalyst. In the present invention, the cross-linkingagent acts as a fluxing agent at reflow soldering conditions and acts asa cross-linking agent in the involved resin system in a manner such thatsoldering is not inhibited by premature gelling.

In systems of the present invention, where electrical connections alonga single axis are involved and reflow soldering is effected in a liquidresin system, the sequencing of stages of cross-linking vis-a-vis reflowsoldering is critical.

It is essential that the gel point of the system not be reached prior toformation of the interconnection by liquid solder which interconnectionoccurs at reflow solder temperature.

It has been observed that where the gel point is reached before meltingof the solder, the solder does not wet and does not effectively engagethe opposing locus where the electrical interconnection is to be madebecause solder flow is restricted.

It is believed that the criticality of this requirement in the system ofthe present invention derives from the inability of the solder to flowin a resin system where the gel point has been reached. It is believedthat where the resin system enveloping the solder is liquid, even wherethe viscosity is great, the melted solder traverses and bonds to theopposing target area and is fluxed by the cross-linking agent. In fact,where a mixed phase exists, wetting, bonding and fluxing occur; however,once the gel point is reached and melt occurs, observation of the curedproduct demonstrates the failure of successful interconnection and thefailure of wetting and flow.

One aspect of the present invention relates to an epoxy resin basedencapsulant-forming composition for use in reflow soldering of anelectrical component to a substrate. The composition forms an acidanhydride epoxy resin system. The fluxing encapsulant composition iscomposed of an epoxy resin, an anhydride cross-linking agent for theresin that also functions as a fluxing agent at reflow solderingconditions and a catalyst. The combination of compounds comprising theencapsulant composition is selected to provide a composition which doesnot reach gel point prior to formation of electrical interconnection atsaid reflow soldering conditions.

The thermosetting resin is preferably an epoxy resin and thecross-linking agent is selected from anhydrides that intrinsicallypossess the added activity of fluxing at reflow soldering temperatures.

Referring specifically to eutectic tin/lead soldering temperatures (183°C.), examples of anhydrides that serve as fluxes include succinicanhydride, methyltetrahydrophthalic anhydride, polyadipicpolyanhydrides, tetrahydrophthalic anhydride, hexahydrophthalicanhydride, polyazelaic polyanhydrides, and admixtures thereof. Thefluxing agent of choice is methyltetrahydrophthalic anhydride and thecatalyst-containing encapsulant composition of choice is tin octoate.

Another aspect of the invention relates to a method of attachment andencapsulation of integrated circuits, such as flip-chips or ball-gridarrays, wherein reflow soldering is effected by the encapsulant-formingcomposition. It is essential that the catalyst promotes gel formationduring or after soldering, but does not form a resin gel (a gel at orpast gel point) before soldering in order not to inhibit soldering byformation of the gel prior to soldering.

Yet another aspect of the present invention relates to an electricalcomponent having a plurality of electrical terminations, eachtermination is composed of a solder bump; a component carrying substratehaving a plurality of electrical terminations corresponding to theterminations of the electrical component; and, an encapsulating materialthat removes metal oxides from the surfaces of the electricalterminations of both the component and substrate. Such encapsulatingmaterial is preferably composed of an epoxy resin, an anhydride thatfunctions as a fluxing agent to remove the oxide coatings from thecomponent terminations and the substrate terminations prior to andduring soldering and also reacts with the epoxy. The gel point of theencapsulant-forming composition is reached at or above the solderingtemperature. In the assembly, the encapsulating material is disposedbetween and fills the opening between electrical component andsubstrate. This may be done either by dispensing encapsulant on theboard and then pressing the component onto the encapsulant or by dippingthe component in the encapsulant prior to placement on the board. Afterheating to soldering temperature, the solder is reflowed andelectrically connects the electrical component to the substrate. Thecatalyst present in the encapsulation composition promotes gel formationduring or after soldering so that soldering is not inhibited bypremature gelling. In other words, fluxing and reflow soldering occurprior to resin system gel point.

Properties of Encapsulant (The Cured Encapsulating Composition)

The physical properties of the cured epoxy encapsulant include suchmeasurable properties as glass transition temperature, tensile strength,modulus, dielectric strength, and dissipation factor. These propertiesaffect reliability of the final encapsulated component. The applicationof the product containing the encapsulated component determines thephysical properties that the encapsulant must possess.

For example, flip-chip components typically require glass transitiontemperatures at or above 120° C. The choice of the resin and anhydridecomponents will therefore be restricted to compositions which yieldglass transition temperatures at or above 120° C. DGEBA resins, EEW180-190 are among the resins of choice for this application.

RESIN SELECTION

The choice of the resin in the system depends on the desired propertiesof the final product. The functionality of the resin and its chemicalstructure will influence the cross-link density of the cured system.Typically, DGEBA or novolak resins are preferred with anhydride curedsystems. Epoxy resins based on methylene dianiline; such as, MY720 fromCiba Geigy or resins containing an amine will cause premature gelationbefore soldering, since the amine acts as a catalyst. Therefore, theselatter epoxy resins are unacceptable for use in the present invention.

The present invention involves the use of compositions which function asflux and hardener for thermosetting resins and in particular epoxideresins, the resins of choice, in encapsulating formulations which findapplication in and are adopted for use at surface mount reflow solderingprofile conditions adapted for assembly architecture and composition ofthe bond sites. The encapsulants of the present invention are formulatedbased on reflow temperature profile and the bond site composition which,after melt, forms the electrical interconnection.

Thus in accordance with the present invention the soldering profile insuch instances includes heating the assembly components to a temperaturebelow the soldering temperature to prevent damage to the assembly partsthat are susceptible to injury at high thermal excursion rates. Thisheating step is dependent on the size, mass and materials used in theassemblies and is referred to in the art as the soak or equilibriumstage of the profile. In connection with computer mother boards,telecommunications equipment and panels of smaller assemblies, thetarget temperature at the end of the soak step preferably approaches thesolder melt point and may range from about 20° C. below the solder meltpoint up to the melt point thereof. The period of application of heat toachieve the target equilibrium temperature typically varies and may befrom about 30 to about 120 seconds for large surface mount assemblies.The soaking step is preceded by a rapid increase in temperature, calledthe ramp step; the rate of increase in temperatures is selected toachieve maximum heat input without injury such as microcracks, warpingand the like and is dependent on the involved architecture andmaterials. In the case of typical large surface mount assemblytemperature increases of about 1° C. to about 4° C. per second prior tosoak are the norm. The final step involves a rapid heating stepfollowing soak where the assembly is quickly heated to a temperatureabove the solder powder melting point in order to form the metalinterconnection between the assembly components. Typically one componentof the assembly is an electrical component such as a packaged orunpackaged integrated circuit and the other component is a substrate formicroelectronic circuitry on which the metal bond sites are located. Inthe art this step is referred to as the spike stage with a total timeabove melt point typically ranging from about 30 to 90 seconds. The peaktemperature at any locus on the assembly is preferably in the 205° C. to235° C. range.

For low thermal mass assemblies; that is, assemblies less than 0.062inches thick and/or assemblies having an area typically 10 square inchesor less, heat input is typically achieved using a smooth non-steppedramp at the rate of 1°-4° C. per second. Solder melt typically isachieved for low thermal mass assemblies in a time frame varying fromsomething less than a minute up to about three minutes.

As the thermal mass of the assembly increases, slower heating rates andstepped processes are employed to bring such assemblies up to thermalequilibrium prior to reflow.

A criteria for selection and combination of thermosetting resin andcross-linking agent (whether in the presence of catalyst or otherwise)in order to achieve the requisite sequence of achieving solder meltbefore the gel point is reached resides in selecting a thermosettingresin and cross-linking agent combination that when heated usingdifferential scanning calorimetry (“DSC”) displays initiation ofexothermic reaction with the range of about 140° C. to 180° C. for leadtin eutectic mixture. Exothermic activity as observed using DSCcorrelates to cross-linking activity. The peak of said exotherm in turncorrelates with the level of cross-linking that indicates gel formation.

Although in the final analysis gel formation for the specificcombination is determined empirically and exceptions occur, the criteriafor selection that is generally applicable is to choose polymer systemswherein exothermic activity, as observed using DSC, is preferablyinitiated at a temperature no lower than 40° C. below the solder meltingpoint and where the peak exothermic activity registered occurs at atemperature above the solder melting point.

Anhydrides, including, in particular, highly active polyanhydrides, arethe preferred dual-functioning cross-linking and flux agents for theencapsulant of the present invention.

In addition to the aforesaid anhydrides, other cross-linking agents thatare suitable for use in the present invention, though not necessarilywith the same order of success, include materials containing chemicalfunctionalities, such as carboxyl, amino, imino, amide, and mercaptan,as described in Angelo, that inherently act as fluxing agents as well ascross-linking agents.

Amine cross-linking agents that provide the requisite latency (gel pointformation at or above the melting point of the solder powder), such asAificure—LX-1 (manufactured by Ajinomoto Co., Inc., Tokyo, Japan), whichis heterocyclic diamine having an active hydrogen equivalent 67, may beused.

Amides and other known nitrogen containing curing agents such asmelamine, dicyandiamide, imidazoles, hydrazides, thioureas and the likemay also be used as cross-linking agents.

Also useful as crosslinking agents are the well-known polydric phenolswhich are polycondensates of one or more phenols such as phenol, variousalkylphenols and naphthols with aldehydes such as formaldehyde,acetaldehyde, acrolein, glyoxal, benzaldehyde, naphthaldehyde andhydroxybenzaldehyde or ketones such as cyclohexanone and acetophenone,vinyl polymerization type polyhydric phenols such as polyvinylphenol andpolyisopropenylphenols, the polyhydric phenols of the present firstinvention, Friedel-Crafts type reaction products of phenols with diolssuch as those represented by the formula (1):

Dialkoxy compounds represented by the following formula (2):

or, dihalogens represented by the following formula (3):

and Friedel-Crafts type reaction products of phenols with diolefins suchas dicyclopentadiene and diisopropenylbenzene.

In assemblies where the inherent fluxing ability of the cross-linkingagent is insufficient to form proper metallurgical solder connections,it may be necessary to add additional fluxing agents in order toincrease the flux activity to the formulation.

To supplement the fluxing activity possessed by the encapsulantattributable to the combination of cross-linking agent and thermosettingresin, other known fluxes may be added in minor amounts so as not toadversely effect the properties of the cured thermoset polymer.Moreover, in choosing the fluxes, whether the basic cross-linking agentsof the present invention with flux activity or the supplemental fluxingagents, low molecular weight compositions which generate outgassing atsurface mount reflow soldering conditions utilized cannot be used sincethe integrity of the encapsulation is compromised when outgassing occursby the presence of voids in the encapsulant. Accordingly, dicarboxylicacid flux agents which produce outgassing should be avoided.

Moreover, in selecting the supplementary fluxing agent, its effect onmaterial properties, such as adhesion, tensile strength, moistureabsorption or glass transition temperature must be considered.Accordingly, compounds appearing to be compounds of choice because ofhigh flux activity, such as dicarboxylic acids, which reduce glasstransition temperature and bulk physical properties of the final polymernetwork, need to be used, if at all, in amounts that do not affect theresultant polymer.

It has been found that the carboxy terminated polyesters used in the inkformulations of U.S. Pat. No. 5,439,164 are suitable as supplementaryflux additives in the encapsulants of the present invention.

It has also been found, where the cross-linking agent, in the amountsused, has inadequate activity as flux to achieve the required level ofoxide removal, that among the preferred additives are those thatcross-link or bind chemically into the polymer matrix and do not retardthe physical properties of the resultant polymer network. For thispurpose carboxy terminated polyesters have been found to be a suitableflux additive.

SUMMARY OF THE INVENTION

The present invention relates to an encapsulant suitable for use atsurface mount reflow profile conditions and a method of electricallyconnecting an electrical component having a plurality of metal bond siteterminations to a component carrying substrate having a plurality ofmetal attachments sites corresponding to the terminations of saidelectrical component, at surface mount reflow profile conditions. Theseconditions vary based on the solder used on the solder bumps, typicallyforming the metal bond site on the electrical component and thecomposition and architecture of electrical component and substrateinvolved. The protocol followed is equivalent to that described in FIG.4 herein. The encapsulant utilized is comprised of a thermosetting resinand a cross-linking agent that acts, below the solder melt temperature,as a fluxing agent which removes oxide coatings from the surface of themetal bond sites on said electrical component and on said componentcarrying substrate when heated to said solder melt temperature. Thethermosetting resin and cross-linking agent combination has a gel pointat or above the solder melt temperature, and preferably above the soldermelt temperature. The method comprises the steps of: a) removing oxidecoating from the surface of metal bond sites while the encapsulant is inliquid form; b) melting the solder at the metal bond sites toelectrically connect the electrical component which has the plurality ofmetal bond site terminations to the component carrying substrate whichhas the plurality of metal bond site terminations corresponding to theterminations of said electrical component while said encapsulant is inliquid form and prior to said encapsulant reaching its gel point; thenc) forming an encapsulant gel; that is, reaching the gel point.Thereafter the gel may be cured to form the encapsulant, therebyproviding protection and enhancing the mechanical connection.

The thermosetting resin is preferably an epoxide. The preferredcross-linking agents are a polyanhydride and carboxy terminatedpolyesters.

The present invention also relates to a method of producing a lowthermal mass assembly by, simultaneously, during surface mount reflowsoldering, encapsulating and electrically connecting an electricalcomponent having a plurality of metal bond site terminations to acomponent carrying substrate having a plurality of metal attachmentssites corresponding to the terminations of said electrical componentutilizing an encapsulant comprised of a thermosetting resin and across-linking agent that acts, below the solder melt temperature, as afluxing agent which removes oxide coatings from the surface of the metalbond sites on said electrical component and on said component carryingsubstrate. The thermosetting resin and cross-linking agent combinationhas a gel point at or above, and preferably above, the solder melttemperature. In this method the low thermal mass assembly is ramped atthe rate of about 1° C. to about 4° C. per second for a time sufficientto achieve solder melt and electrical interconnection. The steps of themethod occur sequentially during ramping, as follows:

-   1. oxide coating is removed from the surface of the metal bond    sites, while the encapsulant is in liquid form;-   2. The solder, which may comprise one or the other bond sites is    melted to electrically connect the electrical component having a    plurality of metal bond site terminations to the component carrying    substrate having a plurality of metal attachment sites corresponding    to the terminations of said electrical component (the encapsulant is    in liquid form during this step and solder melt occurs prior to the    encapsulant reaching its gel point); then-   3. The encapsulant gel is formed at the gel point. Thereafter, the    resultant gel is cured to encapsulate said electrical solder    connections. The preferred thermosetting resin is an epoxide. The    preferred cross-linking agents are polyanhydride and carboxy    terminated polyesters.

Yet another aspect of the present invention relates to a specificencapsulant for encapsulating and electrically connecting a metal bondsite of a first electrical component to a metal bond site of a secondelectrical component at surface mount reflow conditions. The encapsulantcomprises an epoxy resin; a cross-linking agent for the resin that alsoacts as a fluxing agent that removes oxide coatings from the surface ofthe first and second electrical components; and a catalyst forcatalyzing cross-linking of said epoxy resin with said cross-linkingagent, the peak exotherm of said mixture of catalyst, epoxy resin, andcross-linking agent as measured using DSC at a ramp rate of 10° C. perminute being at or above the solder melting point whereby the gel pointof said cross-linked epoxy resin is reached after solder melt. Thepreferred catalyst is tin octoate. The preferred cross-linking agentsare polyanhydrides.

Another aspect of the present invention relates to a method forselecting the components of the encapsulant wherein the encapsulant iscomprised of a thermosetting resin, a cross-linking agent that also actsas a flux and optionally a cross-linking catalyst which comprises: a)identifying the melting point of the involved solder at the metal bondsites; b) heating the encapsulant composition to a temperature exceedingthe melting point of such solder; c) measuring the exothermic activityof the encapsulant during heating step b); d) identifying thetemperature at which peak endothermic activity occurs; and e) selectingas an encapsulant an encapsulant that displays peak exothermic activityat or above the melting point of the solder.

The selection method suitably also includes identification of theinitiation of exothermic activity and selection of the encapsulant alsobased on initiation of thermal reaction. The encapsulant selected is onethat wherein initiation of exothermic activity occurs at a temperaturethat is no lower than 40° C. and preferably no lower than 30° C. belowthe solder melt temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevational view of a device prior to attachmentto a substrate.

FIG. 2 is a sectional elevational view of a device after being reflowedto a substrate.

FIG. 3 is a reflow soldering profile of the soldering method of thepresent invention.

FIG. 4 is a surface mount reflow profile for 63 Sn/37 Pb also applicableto Sn 62/Pb 36/Ag 02 solder illustrating typical parameters where suchsolders are used.

FIG. 5 is the specific profile referred to in Example 4.

FIG. 6 is a graphic representation charting heat change versustemperature of the encapsulant described in Example 1.

FIG. 7 is a graphic representation charting heat change versustemperature of the encapsulant described in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

The catalytically activated encapsulant of the present inventioncomprises (1) thermosetting resin, preferably an epoxy resin, (2) across-linking agent for such resin, which also functions as a fluxingagent during reflow soldering, and (3) a catalyst. The components of theencapsulant are selected and combined to form, in combination, acomposition having a gel point temperature at or above the solderingtemperature whereby surface soldering is not inhibited by gelling priorto soldering when using a surface mount temperature profile. As usedherein, gel point temperature is the temperature for the involvedthermosetting resin system where, after initiation of catalyzedcross-linking, the gel point is reached. Simply stated, the process ofthe present invention will not produce a satisfactory electricalconnection if, prior to solder melt, which occurs at the solder melttemperature, the gel point of the involved epoxy resin system isreached. The cross-linking agent acts as flux during the soldering step.The cross-linking agent cures the epoxy resin and performs this latterfunction paired with a catalyst that catalyzes cross-linking activityand causes gel formation at or above the soldering temperature at whichelectrical connection with solder melt occurs. Other objects andadvantages will become apparent to those skilled in the art from areview of the detailed description of the invention which follows.

In systems of the present invention, the sequencing of stages ofcross-linking vis-a-vis reflow soldering is critical.

It is essential that the gel point of the system not be reached prior toformation of the connection by liquid solder (solder melt) whichconnection occurs at surface mount reflow solder temperature.

It has been observed that where the gel point is reached before meltingof the solder, the solder does not wet and does not effectively engagethe opposing loci to be electrically connected because solder flow isrestricted.

It is believed that the criticality of this requirement in the system ofthe present invention derives from the inability of the solder to flowin a resin system where the gel point has been reached. It is believedthat where the resin system enveloping the solder melt is liquid, evenwhere the viscosity is great, effective fluxing occurs. In fact, where amixed phase exists, wetting, bonding and fluxing occur; however, oncethe gel point is reached and melt formation occurs after the gel pointis reached, observation of the encapsulated assembly demonstrates thatsuccessful connection is not achieved. There is a failure of proper meltformation, wetting and flow.

Referring to FIG. 1, a substrate 100 with metallization pattern 110 isselectively coated with an encapsulating material 120. The material isan encapsulant-forming composition comprised of a thermosetting resin, across-linking agent for said resin, which also acts as a fluxing agentfor reflow soldering and a catalyst selected to provide, in combinationwith resin and flux, a composition in which gelation of the resin doesnot interfere with soldering.

An example of a suitable encapsulating material comprises (1) an epoxyresin, diglycidal ether of bisphenol A with an epoxy equivalent weightof 188, (2) methyltetrahydrophthalic anhydride cross-linking and fluxingagent and (3) tin octoate catalyst. The cross-linking agent is ananhydride cross-linking agent which also acts as fluxing agent. TheMTHPA cross-linking agent is paired with tin octoate catalyst thatcatalyzes cross-linking at a temperature at or above about the solderingtemperature, thereby preventing premature gel formation prior toformation of the electrical interconnections by flow soldering.

The amount of anhydride relative to epoxy, preferably ranges from about75 parts to about 85 parts of anhydride per hundred parts of epoxy resin(75-85 phr).

The amount of catalyst in the composition preferably ranges from about0.1 to about 5 weight percent. In the case of tin octoate, the preferredamount is from about 1.5 to about 2.5 weight percent and the optimalamount is about 2.0 weight percent based on the weight ofencapsulant-forming composition inclusive of catalyst.

A device 130 containing solder bumps 140 is positioned so that thesolder bumps 140 and the active surface 150 are facing the substrate 100and aligned with the metallization pattern 110 of the substrate 100.Referring to FIG. 2, the bumped device 230 is moved into intimatecontact with the metallization pattern 210. The encapsulant-formingcomposition 220 wets the device 230, insuring complete coverage of theactive surfaces 250 of the device 230. The meniscus 260 provides acontinuous seal around the periphery of the device 230 to protect theactive surface 250 from environmental contamination. The cross-linkingagent contained in the encapsulant-forming composition 220 coats thesolder bumps 240 and the metallization pattern 210.

It should be appreciated that although the drawings depict the device130 as an integrated circuit encapsulated and connected to a substrate,embodiments using other types of surface mounted components havingsolder bumps or not are within the scope of the invention.

The assembly 270 is reflowed in a conventional manner; the cross-linkingagent, functioning as flux, reduces the oxides on the solder 240 and themetallization surface 210, and permits alloying of the solder to themetal.

FIG. 3 represents a typical surface mount profile. The present inventioncan accomplish both soldering and encapsulation following such typicalprofile followed by post-cure at substantially lower temperatures thansoldering, in accordance with the profile, typically 156° C. for about 1to 2 hours. Illustrating the requirements of the present invention, byreference to the surface mount profile depicted in FIG. 3, gel formationprior to reflow, Zone 3, would impede soldering. Premature gelationbefore reflow forms a physical barrier preventing the solder fromwetting the target metal surface.

In FIG. 3, Zone 1 depicts the step of preheating in terms of theparameters of the temperature (ordinant) and time period (abscissa) thatthe entire assembly comprising substrate, components and encapsulant ispreheated to a temperature typically 25° C. to 50° C. below the soldermelting temperature. Zone 2 depicts the soak step illustrating theperiod of soak where the temperature of the assembly is allowed toequilibrate. Zone 3 depicts the reflow step when reflow occurs. Zone 4depicts the cooldown step. The surface mount profile illustrated in FIG.3 is merely illustrative of a typical profile which is applicable, interalia, to the embodiment of the invention exemplified in Example 1. Timeand temperature on which soldering is dependent, is dependent on theencapsulant and other parameters of the process.

Selection of Encapsulant System Components for Given Reflow Profiles

As noted herein, the present invention relates to solder pasteformulations and methods of their use specifically adopted for formingprotectively encased isotropic electrical interconnections at reflowprofile conditions used in the manufacture of small and large massassemblies. Such reflow profile conditions require application of heatover time and encapsulant formulation is critical to achievingsatisfactory wetting and solder melt to provide electricalinterconnection at the bond sites as well as satisfactoryencasement/mechanical bonding.

In choosing the encapsulant used in the present invention, selection ofthermosetting resin and cross-linking agents having the dual functionsrequired in the present invention is critical in order to enablesatisfactory electrical and mechanical bonding at the electrical bondsites to take place. The encapsulant requirements and selections arediscussed below.

During the reflow profile, the polymer must not reach its gel pointbefore solder melt, since the molten solder must displace the polymer inorder to wet the bonding surface. If the polymer reaches its gel pointtoo quickly, a polymer barrier is formed between the metal surfaces.Since this gelation inhibits the solder from wetting the substrate metalbond pad, it is critical to understand the curing mechanism and kineticsof the polymer in order to understand the effect on soldering.

In thermoset polymers, the addition of heat initiates the irreversiblereaction between the epoxy resin and the cross-linking agent. During thecure process the epoxy resin molecules react with the cross-linkingmolecules to form long polymer chains and networks with increasingviscosity. As the network grows, a point of infinite viscosity isreached called the gel point. At this point the polymer changes from aviscous liquid to a solid that does not flow.

The gel time of thermosetting resins is usually found under isothermalconditions. For example, epoxy samples can be heated at a settemperature and tested using viscosimetric methods to find the gel time.An example of such a method is to heat epoxy resins in sample pans whilemeasuring resistance to flow.

Since the method of heat during the process described in this inventionis not isothermal but rather involves a slow heating of the sample tosoldering temperatures, it is important to instead estimate the gelationof the polymer by examining the cure kinetics.

Several techniques can be used to examine cure kinetics of the thermosetpolymers. One such method is dynamic mechanical analysis (DMA), whichmeasures the polymer's ability to store and dissipate mechanical energy.Another common technique is to use differential scanning calorimetry(DSC), which measures changes in heat.

As the chemical reaction of thermoset resins during polymerization isexothermic, this change in heat can be measured using DSC and related tothe extent of chemical reaction. As described by Hadad in Epoxy Resins.Chemistry and Technology, May, ed., Marcel Dekker, 1988, p. 1130, “anassumption is made that the amount of energy given off during the cureis proportional to the extent of chemical reaction.” Using techniquesdescribed by Hadad it is possible to estimate the kinetic activationenergy required for initiation of polymerization. One such methodinvolves generating DSC scans using different heating rates as describedin ASTM Method E 698-79. However, since the critical gelation point inorder to prevent proper soldering interference as described in thisinvention must be determined empirically, DSC will be shown to serveonly as a guide for proper material combinations as the solderingprocess has previously been described as ranges. Therefore, a single DSCscan at a single heating rate is used to show the relationship betweencure kinetics of thermosetting polymers and their applicability for usein the method of soldering described in this invention.

The following examples serve to illustrate a mode of practicing theinvention.

EXAMPLE 1

An encapsulating material of the present invention with fluxingproperties was prepared by combining the following components:

Component % by Weight Shell 828 (DGEBA − EEW = 188) 54.5 A&C AC220J(MTHPA) 43.5 Catachek 860 (tin octoate) 2

The material was spread on the copper surface of a standard copper cladFR-4 board. A small ring of 63 Sn/37 Pb eutectic solder was placed onthe epoxy. The board was placed in an IR reflow oven. The profile wasthe standard solder paste reflow used for solder paste. The ringsoldered to the copper leaving an epoxy residue which was past its gelpoint.

EXAMPLE 2

The encapsulating material prepared is dispensed on the surface of themetal, copper, which has been plated to an organic substrate, such asFR-4. A solid piece of solder, 63 Sn/37 Pb eutectic, is placed on thesurface of the epoxy. The test board, now containing both theencapsulating material and the solder on the surface of the metal, areplaced in a reflow oven, either IR or convection and subjected to thestandard reflow profile shown in FIG. 3. The epoxy material serves firstas a flux and assists in the bonding of the solder to the copper. Afterthe reflow, the epoxy is gelled, that is, proper interconnection isobserved.

EXAMPLE 3

The encapsulating material prepared in Example 1 is dispensed on themetal bond sites on substrates to which a solder bumped component, suchas a flip-chip, would attach. The flip-chip die is placed on theencapsulating material so that the encapsulating material completelyinterfaces the lap between the die and the substrate, the substrate canbe organic, such as FR-4, or inorganic, such as, glass or ceramic. Themetal bond sites are copper or gold, with or without a plating ofsolder. The solder bumps on the die are 63 Sn/37 Pb eutectic, on whichthe solder from the die bumps reflows and bonds to the bond sites on thesubstrate. Suitably a high melting alloy, such as 63 Sn/37 Pb, can beused, in which case the eutectic solder plating on the metal bond sitewould reflow and bond to the bumps. On the die, the flip-chip die,encapsulating material and substrate are aligned and placed in a reflowoven, either IR or convection, and subjected to the standard reflowprofile shown in FIG. 3. During the reflow process, the MTHPA componentof the encapsulating material serves as a flux and enables the solderingof the component to the board. The pairing of MTHPA and tin octoate alsoprovides an appropriate level of cross-linking during the reflowprocess, which yields a resin gel suitable for post cure and wherein thegel point is reached at a point in the overall process where solderingis not inhibited.

The following examples serve to illustrate a mode of practicing theinvention including selection of encapsulant components.

EXAMPLE 4

A mixture of a bisphenol A epoxy resin (Shell Epon 828), methyltetrahydrophthalic anhydride (MTHPA) (Lonza AC220J) and the catalystTris(dimethylaminomethyl)-phenol, commonly referred to as DMP-30 (LonzaAC-30) was prepared according to formulations as described in literatureavailable from the chemical suppliers:

Epon 828 55 parts MTHPA 43 parts DMP-30  2 parts

The resultant composition was spread onto a copper heated to 250° C.along with a slug of 63Sn/37Pb solder in accordance with the method setforth by Angelo (U.S. Pat. No. 3,791,027) and in heated rapidly asdescribed in Pennisi U.S. Pat. No. 5,128,746 and Capote U.S. Pat. No.5,376,403. It was observed that the anhydride (MTHPA) cleaned thesurface oxides from the metal surfaces which allowed the solder to wetonto the copper and form a metallurgical interconnect.

This same mixture was then spread onto the metal surface of an FR-4epoxy glass substrate plated with copper along with a slug of 63Sn/37Pbsolder. The substrate, containing the epoxy composition and the solderslug, was placed into an IR reflow oven with multiple heating zones. Thetemperatures of the zones and the belt speed of the oven had previouslybeen profiled to yield substrate surface temperature profile as shown inFIG. 5.

Using this temperature profile the solder slug did not wet the coppersurface on the FR-4 and form a metallurgical connection. Instead thesolder slug remained in its original shape and was encapsulated in thecured epoxy.

In addition to the empirical soldering tests described above, DSC wasused to examine the cure kinetics of the above epoxy. Using a 50 mgsample and a heating rate of 10° C./min from 50° C. to 230° C., theonset of the cure exotherm was observed at 100° C. while the peakexotherm was observed at 130° C., as shown in FIG. 6. Therefore, using aheating rate of 10° C./min., the maximum exotherm of the cure reactionoccurs 50° C. below the melting point of the solder.

EXAMPLE 5

Using the same resin and anhydride combination as in Example 4, LonzaACPI (a proprietary imidazole) was used as a catalyst.

Epon 828 55 parts MTHPA 43 parts ACPI  2 parts

This mixtures was used with solder slugs heated rapidly on copper. Theanhydride cleaned the metal oxides and the solder wet and spread ontothe copper surface. When the composition was spread onto copper overFR-4 organic substrate and heated in the multiple zone heating oven,however, the result was the same as described in Example 4. The solderdid not change shape and did not spread onto the metal surface.

The DSC scan from 50° C. to 230° C. at 10° C./min on a 50 mg sampleshowed the exotherm onset to begin at 100° C. and peak at 130° C. Again,although rapidly heating the combination of materials allowed the solderto spread onto the copper, when heated slowly to solder melt point usinga multiple zone oven the epoxy system described did not have thenecessary latency to remain liquid before soldering.

Similar results were observed using the Epon 828 and MTHPA combinationwith several other known anhydride cured epoxy catalysts, includingDimethylaminomethylphenol (Lonza AC-10), zinc octoate (ShepardChemicals), Benzyldimethylamine (Lonza BDMA), Diazabicycloundecene (AirProducts Amicure DBU-E), 2-ethylhexanoic acid salt ofDiazabicycloundecene (Air Products Amicure SA-102),2-heptadecylimidazole (Air Products Curezol2MA-OK),2-Heptadecylimidazole (Air Products C17Z),2,4-Diamino-6(2′methylimidazoleyl-(1′))ethyl-s-tri (Air Products 2MZAzine).

In each case the catalyst was used with a bisphenol A resin (EEW 190)such as Shell Epon 828 and Methyl Tetrahydrophthalic anhydride at 80parts based on the weight of the resin. Using the catalysts stated aboveat 1-5 parts based on the weight of the resin, the mixtures were foundto clean the metal oxides and promote spreading of 63 Sn/37 Pb solder tocopper when heated rapidly, but cured too quickly and inhibitedsoldering when heated slowly in a multizone conveyorized heating oven asdescribed in Example 4.

EXAMPLE 6

Stannous octoate (Ferro Bedford Catachek 860) was selected as a catalystfor the bisphenol A resin, MTHPA composition as described in example 1.

Epon 828 55 parts MTHPA 43 parts Stannous octoate  2 parts

This mixture was used with solder slugs heated rapidly on copper. Theanhydride cleaned the metal oxides and the solder wet and spread ontothe copper surface. When the composition was spread onto copper overFR-4 organic substrate and heated in the multiple zone heating oven, thesolder spread onto the metal surface of the copper and the epoxy waspartially gelled. Thus the catalyst selection with this combination ofresin and crosslinking agent provided the latency for gelation to takeplace after soldering.

The DSC scan from 50° C. to 230° C. at 10° C./min on a 50 mg sampleshowed the exotherm onset to begin at 150° C. and peaked well above 200°C., as shown in FIG. 7. Thus, the peak exotherm, which is used topredict latency, occurs well above the solder melt point of 183° C.

EXAMPLE 7

Using stannous octoate as a catalyst with MTHPA along with amultifunctional resin such asN,N,N′,N′-Tetraglycidal-4,4′-methylenebisbenzenamine (Ciba Geigy MY720).This tetra-functional resin has a higher reactivity than one based onbisphenol A. A mixture of MY720 with MTHPA at 90% of the stoichiometricratio was prepared with stannous octoate used as the catalyst and addedat 1 part based on the weight of the resin.

MY 720 45 MTHPA 54 Stannous octoate 1Using this composition in the rapid heating method described in Example1 the solder spread and formed a metallurgical connection to the copper.However, when using the heating method involving the multizone oven thesolder did not spread or wet onto the copper. Eliminating the stannousoctoate and using the MY70/MTHPA combination described showed the sameresult. The solder was inhibited from wetting the copper when heatingthe sample in the multizone oven.

When cycloaliphatic resins such as3,4-Epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (UnionCarbide ERL-4221) are used in combinations with MTHPA are used withstannous octoate as the catalyst the soldering result observed issimilar to Example 1. The composition cures before solder melt point andinhibits the spread of the solder to the copper.

A DSC scan of the composition: ERL-4221 49 parts MTHPA 49 parts StannousOctoate  2 partsat a heating rate of 10° C./min showed the exotherm onset at 90° C. witha peak exotherm at 155° C. Thus, the exotherm peak, which correlates tothe chemical reaction rate, is lower than the melting point of thesolder alloy.

A blend of epoxy resins Epon 828 and ERL 4221 was used with MTHPA andstannous octoate as a catalyst.

ERL-4221 24.5 Epon 828 24.5 MTHPA 49 Stannous Octoate 2This system solder soldered 63 Sn/37Pb alloy to copper when heatedrapidly but did not solder using the multizone heating oven. The DSConset temperature began at a higher temperature of 120° C. compared tothe composition using only the cycloaliphatic resin (ERL-4221). Althoughthe peak exotherm was nearly identical in temperature to the singleresin system yet the height of the exotherm was lower with the resinblend system indicating less heat given off and therefore less chemicalactivity. Thus, blends of highly reactive resins such as cycloaliphaticresins with MTHPA do not provide the necessary latency.

EXAMPLE 8

A blend of bisphenol A system, MTHPA and a dicyandiamide type catalyst(Ajinomoto Ajicure AH-150) was used at various catalyst levels.

Epon 828 52-56 parts AH-150 40-43 parts AH-150  1-8 partsUsing catalyst levels of 1-2 parts based on the weight of the resin werefound to solder the 63 Sn/37 Pb alloy both using rapid heating as wellas using the multizone oven. However, when used at concentrations above3 parts based on the weight of the resin the compositions inhibited thesolder from spreading using the multi-zone heating profile. Thus, thecatalyst level is critical to the rate of reaction. DSC exothermsrevealed that the onset of the peak exotherm occurs at lowertemperatures as catalyst concentration increases.

The Reflow Profile

The cross-linking catalyst (accelerator) selection is based on the needto inhibit gelation during the soldering process. A most importantembodiment of the present invention involves the soldering method knownas reflow soldering. In reflow soldering, the components to be solderedto the circuit board or substrate are placed onto the PCB or substrateand such assembly is passed through an oven containing different heatingzones. Three zones are typical. The first zone, or heating stage, iscommonly called the ramp. In this zone, the surface temperature of theboard is quickly raised to heat the materials of the board andcomponents. In the second zone, known as the soak, the temperature ofthe board is allowed to equalize at this temperature range for a periodof time that may be of short duration (1-3 minutes) in order to reducetemperature differentials on the surface of the board and components.The soak time will depend on the type of board, solder and componentsused. The third step involves a quick temperature spike to exceed thesolder melt point. Therefore, the solder melting temperature dictatesthe temperatures of the zones. Since melting point of 63 Sn/37 Pb is183° C., the soak temperature is generally 120-160° C. The peaktemperature at reflow is generally 200-300° C.

Catalyst

When the encapsulant of the present invention includes a catalyst, theselection of the catalyst is critical and is based on the ability of thecatalyst in the epoxy system to catalyze the cross-linking to the pointof gelation (gel point) during or after the reflow process, withoutinhibiting reflow soldering.

By way of explanation, an uncatalyzed combination of a bisphenol A resincombined with a phthalic anhydride derivative, such as MTHPA, when usedin a reflow soldering process, would not inhibit soldering but wouldalso not reach its gel point during the reflow process. Such systemswould require 6-48 hours of post-cure below soldering temperature(typically 150° C.) in order to reach gelation and fully cross-link.Therefore, a catalyst is necessary to reduce the overall time of cure ofthese systems. However, it has been observed that, if a catalyst isselected which is too active, the system will reach gelation below thesoldering temperature and inhibit the soldering process by forming aphysical barrier between solder and target metal.

Anhydride catalysts of the amine type have shown this detrimentalphenomenon. Examples of unacceptable catalysts are benzyldimethylaminedimethylamino methyl phenol, tris (dimethylamino methyl) phenol,triethanolamine and monoethanolamine. When these amines are used ascatalysts together with bisphenol A epoxy resins, such as shell EPON828, and phthalicanhydride derivatives such as MTHPA using the standardreflow protocol, premature gelation results which inhibits propersoldering.

Other common anhydride-cured epoxy catalysts have been shown to bedetrimental to soldering, as detailed in Example 5, when using blends ofbisphenol A resins and MTHPA.

Tin octoate, the catalyst of choice for these systems, is a metal saltof tin and 2-ethylhexoic acid. Although not wishing to be bound, it isbelieved that the activity of the catalyst arises from the oxidation ofthe tin from Sn⁺² to Sn⁺⁴ and the dissociation of the tin from the2-ethylhexoic acid. When tin octoate is tested using differentialscanning calorimetry, an exotherm is observed at 185-190° C. It has beenobserved and therefore it is believed that the temperature at which theexotherm occurs correlates to the temperature at which catalysis ofcross-linking occurs. Accordingly, one method of evaluating onsets ofcatalytic activity where the mechanism of catalysis of the catalystbeing evaluated corresponds to that of tin octoate is to correlate theexotherm of the catalyst with the temperature at which catalysis ofcross-linking occurs.

When used in combinations with MTHPA and bisphenol A resins stannousoctoate has been shown to effectively provide the required latencynecessary to prevent premature gelation of polymer before soldering.

The choice of stannous octoate as an accelerator (cross-linkingcatalyst) prevents significant gelation of the epoxy during the ramp andsoak stages to allow the soldering to take place when using a reflowtemperature profile for Sn/37 Pb solder or alloys with similar meltingpoints.

Another example of a suitable encapsulant, comprises (1) an epoxy resin,diglycidal ether of bisphenol A with an epoxy equivalent weight of185-192, (2) MTHPA cross-linking and fluxing agent and (3) tin octoatecatalyst. The cross-linking agent is an anhydride cross-linking agentwhich also acts as fluxing agent. The MTHPA cross-linking agent ispaired with tin octoate catalyst that catalyzes cross-linking at atemperature at or above about the soldering temperature, therebypreventing premature gel formation prior to formation of the electricalconnection(s) during reflow soldering.

The amount of catalyst in the composition preferably ranges from about0.1 to about 10 weight % based on total weight of encapsulant. In thecase of tin octoate, the preferred amount is from about 2.5 to about 7weight percent and the optimal amount is about 5 weight % based on thetotal weight of the encapsulant.

Optional Additives

A component which can optionally and advantageously be included in theencapsulant of the present invention is a surface tension reducingagent. It is used to reduce the contact angle at the bonding surfaces.The surface tension reducing agent may be a surfactant. Among thesuitable surfactants are TWEEN®, available from ICI, Wilmington,Delaware, and potassium perfluoroalkyl sulfonates. When present, thesurface tension reducing additive is preferably added in amounts of fromabout 0.1 weight % to about 1 weight % based on the total weight ofencapsulant.

Another component that optionally is added to the encapsulantcomposition of the present invention is an adhesion promoter which hasthe ability to enhance epoxy to metal bonding. Suitable adhesionpromoters include organo silanes and titanates. A suitable organosilaneis 6040, 3-glycidoxy-propyltrimethoxysilane available from Dow CorningCorp. of Midland, Mich. A suitable titanate is LICA 38, neopentyl(diallyl)oxy, tri(diooctyl)pyro-phosphatotitinate available from KenrichPetro Chemicals, Inc., in Bayonne, N.J. The adhesion promoter ispreferably added in amounts of from about 0.1 weight % to about 1%weight % based on the total weight of the encapsulant.

Yet another component that can optionally be used in the encapsulatingcomposition of the present invention is a defoaming agent such as FOAMBLAS™ 1326, an alkoxylate of fatty esters available from Ross Chemicals.The defoaming agent is preferably added in amounts of from about 0.1weight % to about 1 weight % based on total weight of based on the totalweight of the encapsulant.

It is not intended to limit the encapsulant, surface mount reflowsoldering method of manufacture, encapsulant selection and compositionand assemblies of the present invention to the particular embodimentsdescribed herein, and various modifications may be made, including, butnot limited to, changes in the surface mount reflow profiles based onvariations in the solder and hence solder melt temperature andconditions of solder melt, the dimensions, shape and materials, withoutdeparting from the scope and spirit of the invention as set forth in thefollowing claims.

1. An encapsulant for electrically connecting a metal bond site of afirst electrical component to a metal bond site of a second electricalcomponent with solder and for forming an encapsulant encased electricalsolder connection between said first and second components at solderreflow conditions, comprising: a) a thermosetting epoxy resin; b) across-linking agent for said resin that also acts as a fluxing agentthat removes oxide coatings from the surface of said first and secondelectrical components below the solder melt temperature of said solder,and c) a tin octoate catalyst for catalyzing cross-linking of saidthermosetting resin with said cross-linking agent, the peak exotherm ofsaid mixture of catalyst, thermosetting resin, and cross-linking agentas measured using DSC at a ramp rate of 10° per minute being at or abovethe solder melting point whereby the gel point of said cross-linkedthermosetting resin is reached after solder melt, wherein the exothermicactivity as measured using DSC is initiated at a temperature no lowerthan 40° C. below the solder melting point.
 2. The encapsulant of claim1, wherein the cross-linking agent is a polyanhydride.
 3. Theencapsulant of claim 1, wherein the resin is a bisphenol A epoxy resin.4. The encapsulant of claim 1, wherein the cross-linking agent is amethyltetrahydrophthalic anhydride.
 5. The encapsulant of claim 1wherein the cross-linking agent may be blended with a fluxing agent.